This invention relates generally to radio communications systems and more particularly to radio communications systems having multiple nodes and multiple channels assignable among the multiple nodes.
As is known in the art, the bandwidth allocated to a radio communications system may be divided into channels. Each channel provides for a two-way call between a base station and a mobile station. For example, each channel may comprise a pair of modulated carrier frequencies, one frequency for each direction of communication (mobile-to-base station and base-to-mobile station). Because the number of channels allocated to a radio communications system is finite, it is desirable to increase the capacity of the system by allowing multiple calls to use the same channel. This is called channel reuse.
One way to provide for channel reuse in a radio communications system is to divide a geographic territory covered by the system into multiple areas or xe2x80x9ccells.xe2x80x9d In a cellular communications system, for example, each cell has a base station that communicates with the mobile stations within the territory of the cell. Ideally, calls between a base station and the mobile stations within the base station""s cell would not interfere with calls being made within any other cell. In such a case, each channel allocated to the system as a whole could be reused for each one of the cells. That is, two neighboring, non-interfering cells can use the same channel simultaneously. Thus, in the ideal case, the capacity of the system increases in direct proportion to the number of cells in which the system is divided.
In practice, however, interference between neighboring or nearby cells simultaneously using the same channel prevents the ideal case from being realized. The severity of this xe2x80x9cco-channelxe2x80x9d interference depends on the way in which the allocated bandwidth is divided into channels, the proximity of the cellular base stations, which is generally dictated by the size of the individual cells, and the variability of the local terrain. If co-channel interference is too great, steps must be taken to prevent the simultaneous use of channels by base stations in close proximity to each other.
One commonly used method for providing for some channel reuse in a system with co-channel interference is to pre-assign individual channels among the base stations such that no two base stations within a predetermined distance (called the xe2x80x9creuse distancexe2x80x9d) of each other may use the same channel. Such systems are said to use a static channel assignment (SCA) scheme because the channel assignment is fixed. When a call request is made, the base station assigns the call to one of the channels pre-assigned to operate in that cell. In analog systems, such as AMPS, the assigning may be performed in the serving base transceiver station (BTS). In digital systems, such as GSM, the assigning may be handled by the base station controller (BSC). One benefit of such schemes is that they may be implemented relatively simply. However, the ease of implementation comes at a cost. The systems cannot easily adapt to changing user demands over time. Once the pre-assigned channels within a cell are in use, further requests for service from the mobile stations (i.e., users) will be delayed or denied; channels assigned to neighboring cells may not be borrowed to accommodate the demand.
More efficient channel assignment schemes than the SCA schemes commonly used have been proposed in the literature. In one type, called a dynamic channel allocation (DCA) scheme, channels are dynamically assigned among the cells from the full complement of available channels to the system as a function of the operating radio environment existing at the time of assignment. The schemes are dynamic in the sense that the channel assignment can adapt to the changing radio environment and the changing demand on the system as a whole. Generally, these schemes assign channels as a function of the measured carrier to interference ratio (C/I). Accordingly, these schemes generally require scanning hardware throughout the system to monitor the radio environment and require a mechanism to gather and process the data received from the scanning hardware.
Another type of scheme is called flexible channel assignment (FCA). FCA allocates a fixed subset of channels among the various cells much like the SCA scheme and reserves a smaller subset of channels for emergency allocation as needed. Allocation of the emergency channels occurs in a scheduled or predictive manner when a cell or a group of cell""s fixed subset of channels becomes inadequate.
In accordance with the invention, a system and method are provided for efficiently assigning a plurality of channels among a plurality of nodes in a radio communications system. For each node in the system, a buffer zone is established. The members of a particular node""s buffer zone are that subset of the plurality of nodes in the system that are prohibited from using a channel simultaneously with the particular node. When a particular node is in need of a channel assignment, the system assigns a channel to the particular node from a subset of channels that excludes channels assigned to nodes within the particular channel""s buffer zone.
In accordance with one feature of the invention, a node is selected for.channel assignment. A subset of a plurality of channels is then determined. The subset of channels excludes channels assigned to nodes associated with the selected node""s buffer zone.
In accordance with another feature of the invention, channels are assigned to nodes from the full complement of channels available in the system.
In accordance with another feature of the invention, the buffer zones are determined at a time before any channels are assigned (i.e, prior to normal operation). The need to continuously monitor the radio environment is thereby eliminated.
In accordance with another feature of the invention, the maximum size of any node""s buffer zone is selected as a function of the worst-case reuse distance in the system.
In accordance with another feature of the invention, the channel assigned to any particular node is the channel in the subset of available channels which minimizes a predefined cost function.
In accordance with yet another feature of the invention, a co-channel zone is established in addition to the buffer zone. The co-channel zone comprises the subset of nodes within a predetermined distance of a particular node with the restriction that such subset of nodes is not a member of the nodes in the particular node""s buffer zone. When a particular node is in need of a channel assignment, the system assigns the channel that is not assigned within the particular node""s buffer zone and that has minimized the cost function associated with the co-channel zone.
In accordance with yet another feature of the invention, a cellular or PCS system is provided. The cellular or PCS system includes a plurality of channels assignable among a plurality of cells. The plurality of cells each have an associated buffer zone, the buffer zones having been determined and stored in the system""s memory before the system is placed in operation. During operation, when a cell is in need of a channel, a channel is selected from that subset of channels not already assigned to cells within that particular cell""s buffer zone.
In accordance with yet another feature of the invention, a method for use with a radio communications system is provided. With such method, each node has an associated buffer zone. The method includes identifying a one node of a plurality of nodes in the system, selecting a channel from a plurality of channels, checking if the selected channel is assigned to at least one of the plurality of nodes within the selected one node""s buffer zone, repeating the selecting and checking if the selected channel is assigned to a node within the one node""s buffer zone, and assigning the selected channel if the selected channel is unassigned to every node in the selected one node""s buffer zone.
In accordance with yet another feature of the invention, a radio communications system is provided. The system includes a controller for assigning a plurality of channels among a plurality of nodes. The controller includes a memory having stored tables associating each of the plurality of nodes with xe2x80x9cbuffer zonesxe2x80x9d. The controller correlates the plurality of channels to the plurality of nodes. The controller includes a processor for associating a subset of a plurality of channels with a particular selected node, the subset excluding channels assigned to one or more nodes within a stored subset of nodes correlated to the particular selected node, and that assigns one of the subset of channels to the selected node.
These and other features of the invention , as well as the invention itself, will become more readily apparent from the following detailed description when taken together with the accompanying drawings, in which:
FIG. 1 is the topology for a portion of a mobile radio system according to the invention;
FIG. 2 is a block diagram of a cellular system according to the PRIOR ART;
FIG. 3 is a detailed block diagram of the base stations and MSC shown in the radio system of FIG. 1;
FIG. 4 shows exemplary tables reflecting buffer zones and channel usage of the system of FIG. 1;
FIG. 5 shows the system of FIG. 1 reconfigured as a schematic topology of a cellular system according to the invention in a hexagonal arrangement and illustrates the worst-case frequency reuse distance at a cell boundary;
FIG. 6 shows a flow diagram for determining the size of the buffer zone shown for the systems of FIGS. 1 and 5;
FIG. 7 shows a flow diagram for assigning a channel to a particular node according to the invention.